The present invention relates to X-ray imaging apparatus, and more specifically to means of depressing transient current surges through an X-ray tube of the apparatus.
The X-ray imaging apparatus includes a vacuum tube, having a cathode and anode, which emits X-rays during operation. The cathode comprises tungsten thermionic emitting source and focusing surfaces. Upon application of an applied potential the thermionically emitted electrons traverse the vacuum gap between the cathode and anode, impacting the anode thereby generating X-rays.
Occasionally, during normal operation, the vacuum tube will break down, resulting in an abrupt drop in anode to cathode impedance and the passage of a large current through the tube. Over a period of time, repeated breakdowns cause a deterioration of the high voltage electrodes and a reduction in the ability of the X-ray tube to withstand the high bias voltage needed to produce X-rays. Although the tube's power source, or generator, under normal operating conditions has the capability of regulating the flow of current between the anode and cathode, this control is lost during a breakdown.
In a typical medical diagnostic X-ray installation, the X-ray tube and its associated imaging components are often located in one room and the high voltage power generator in another. These components are interconnected by high voltage cables which may be up to 100 feet in length. Usually one shielded cable connects the cathode to a negative (with respect to ground) high voltage supply, and another shielded cable connects the anode to a positive (with respect to ground) high voltage supply. Normally these cables serve as passive elements transferring power between the supplies and X-ray tube, but due to the characteristics of the cable reactance, the energy stored in them can be substantial.
However, in the case of a tube breakdown, the high voltage cables actively participate in the event. Since the cables have significant capacitance and inductance, the breakdown initiates an oscillatory discharge between the anode and cathode cables which now are electrically connected through the low impedance path of the X-ray tube. High transient currents are limited only by the cables surge impedance rather than by limiting impedances of the power source. The electrical discharge, which results from the breakdown, has a periodically changing polarity and damages both the cathode and anode surfaces within the tube. The discharge eventually is extinguished, either by damping which results from resistive elements in the cables or by extinction of the discharge conducted through the gap during a zero current crossing of the oscillation. The resulting damage can greatly impair the ability of the X-ray tube to withstand normal voltages during subsequent operation.